Capacitive accelerometer

ABSTRACT

An accelerometer has a capacitor detect plate defined on an electrically insulating substrate adjacent a capacitor source plate connector and has circuit paths on the substrate connected to the detect plate and source plate connector. A resilient metal plate has an attachment portion secured to the source plate connector, has a capacitor source plate portion, has integral first beam elements extending away from the attachment portion and has a second beam element extending from the first beam elements back toward the attachment portion to dispose the capacitor source plate in spaced relation to the detect plate to form a capacitor having selected initial capacitance. The capacitor source plate member portion is movable relative to the detect plate to modify device capacitance in response to acceleration. Electronic components are mounted on the substrate connected to the circuit paths to provide an output signal corresponding to the acceleration. Preferably the device is mounted on a housing base with a part of the device substrate carrying the metal member extending over a housing recess to isolate that device end from thermal stresses in an accelerometer unit. Preferably the detect plate, source plate connector, resilient metal member, are provided on and soldered on a plurality of detachable sections of a ceramic insulating substrate in a batch process, and the substrate sections are then separated to form a plurality of accelerometer devices.

BACKGROUND OF THE INVENTION

The field of this invention is that of accelerometers, and the inventionrelates more particularly to accelerometers which are adapted to bemanufactured with high reliability and uniformity at low cost for use inlarge volume applications.

Capacitive accelerometers and the like of various types as shown in U.S.Pat. No. 4,483,194 to Rudolph, U.S. Pat. No. 4,435,737 to Colton, RE31,459 to Block, and U.S. Pat. No. 3,240,073 to Pitzer are commonly usedor proposed for use in aircraft applications and the like where theaccelerometers are likely to be subjected to shock, vibration and severetemperature changes but where the accelerometers are required to displayreliable and accurate performance characteristics. However, many ofthese applications are poorly served with such accelerometers or forcetransducers either because the devices are manufactured with limitedperformance capability or at excessive cost. It would be desirable if acapacitive accelerometer could be provided with desired reliability andresponsiveness while being manufactured with uniformity at low cost.

BRIEF SUMMARY OF THE INVENTION

It is an object of this invention to provide a novel and improvedaccelerometer device; to provide such an accelerometer device which ismounted securely and reliably in a sealed accelerometer unitparticularly adapted for use in vehicle applications; to provide such anaccelerometer device or unit which is characterized by reliability andresponsiveness; to provide such an accelerometer device or unit which isadapted to be manufactured with uniformity at low cost; and to providenovel and improved methods for manufacturing such accelerometers.

Briefly described, the novel and improved accelerometer device of theinvention comprises an electrically insulating substrate of a ceramic orother rigid support material such as is conventionally used in printedcircuit boards and the like. An electrically conducting surface layersuch as a copper or other metal film metallization or the like isprovided on the substrate to define a fixed capacitor plate or detectplate on the substrate and also to define a capacitor source plateconnector on the substrate at a location closely adjacent to thecapacitor detect plate. Preferably the electrically conducting layeralso defines circuit path means on the substrate which are connected tothe capacitor detect plate and the source plate connector and which arepreferably arranged to receive electrical circuit components thereon andto be connected in an electrical circuit.

An electrically conductive metal plate member of stiffly resilient,preferably low expansion, metal such as the materials commonly calledKovar, Invar or Alloy 42 or the like is provided with an attachmentplate portion, a capacitor source plate portion, and with integralresilient beam member which extend between the attachment plate andsource plate portions of the member. Preferably the beam means comprisea pair of resilient beam elements which extend in parallel relation toeach other from the attachment plate portion of the member--preferablyfrom respective ends of the attachment plate portion--to dispose distalends of the pair of beam elements at a selected location spaced from theattachment plate portion of the member. A second beam element isconnected to the distal ends of the first pair of beam elements andextends back toward the attachment plate portion of the member tosupport the capacitor source plate adjacent the attachment plate portionof the member. This closely spaces the source plate relative to theattachment plate location for achieving more uniform, repeatable initialcapacitance from device to device in volume manufacture and minimizesthermally responsive movement of the source plate relative to theattachment plate and therefore relative to the detect plate, thereby tominimize thermal drift of the device.

The attachment plate portion of the metal member is secured to thesubstrate in electrically conductive relation to the capacitor sourceplate connector means provided on the substrate to dispose the capacitorsource plate portion of the member in selected spaced relation to thefixed or detect capacitor plate on the substrate, thereby to form acapacitor having a selected initial capacitance and to permit movementof the capacitor source plate of the member in response to accelerationforces to modify that capacitance as a function of the acceleration. Inone embodiment of the invention, the metal plate member is flat and ashim means is soldered, brazed, welded or epoxied between the attachmentplate portion of the member and the source plate connector means tomount the source plate member portion in the desired spaced relation tothe detect plate on the substrate. In another preferred embodiment, themetal plate member is etched or otherwise reduced in thickness,preferably at the top and bottom, within the capacitor source plateportion of the member so that, when the attachment plate member portionis secured to the source plate connector on the substrate, the capacitorsource plate has the desired spacing relative to the detect plate. Ifdesired, the metal member is secured to the source plate connector meansby rivet means or other conventional means. Preferably stop means arealso mounted on the source plate connector means to extend over thecapacitor source plate portion of the metal member in spaced relationthereto to limit movement of that source plate member portion away fromthe detect plate when the device is subjected to shock or otheracceleration forces. Preferably a cover is also sealed to the substrateover the metal member to exclude particles or other extraneous materialsfrom the member area.

Electronic components such as integrated circuit means or the like arepreferably connected to the detect plate and source plate connectormeans, preferably by being mounted on the device substrate, to providean output signal from the accelerometer device which corresponds to theacceleration force or change in the device capacitance withacceleration. Preferably the accelerometer device is mounted in acovered housing of a thermally insulating material or the like and issealed within the housing to be protected from the environment in whichthe assembled accelerometer unit is to be used. Preferably theaccelerometer device has one end of the device substrate mounted on abase portion of the unit housing so that the opposite end of thesubstrate accommodating the detect and source plate of the devicecapacitor extends over a housing base recess, thereby to isolate the endof the device carrying the metal member from thermal stresses in thehousing.

In accordance with the method of the invention, the substrate isprovided in the form of a sheet material having a plurality ofdetachable sections, the electrically conducting means are deposited onall of the sections at the same time, and metal members are alsodisposed on each of the substrate sections together with electronicmeans and simultaneously soldered or secured to the substrate sectionsbefore the sections are separated to form individual accelerometerdevices of uniform properties.

DESCRIPTION OF THE DRAWINGS

Other objects, advantages and details of the accelerometer unit, deviceand method of the invention appear in the following detailed descriptionof preferred embodiments of the invention, the detailed descriptionreferring to the drawings in which:

FIG. 1 is a section view along an axis of the accelerometer unit of theinvention;

FIG. 2 is a top plan view to enlarged scale of a substrate used in anaccelerometer device incorporated in the accelerometer unit shown inFIG. 1;

FIG. 3 is a top plan view to enlarged scale of the accelerometer deviceof the invention incorporating the substrate of FIG. 2;

FIG. 4 is a partial section view to enlarged scale along line 4--4 ofFIG. 3;

FIG. 5 is a partial section view similar to FIG. 4 illustrating analternate embodiment of the accelerometer device of the invention;

FIGS. 6 and 6A are partial section views similar to FIG. 4 illustratingother alternate embodiments of the accelerometer device of theinvention;

FIG. 7 is a top plan view similar to FIG. 2 illustrating anotheralternate embodiment of the accelerometer device of the device;

FIG. 8 is a block diagram illustrating the method of the invention formaking the accelerometer device; and

FIG. 9 is a plan view of a plurality of the accelerometer devices of theinvention illustrating a step in the method of FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, 10 in FIG. 1 indicates the novel and improvedaccelerometer unit of the invention which is shown to include anaccelerometer device 12 mounted within a unit housing 14 and sealedwithin the housing by a cover 16 using a sealant 16.1. Preferably thehousing and cover are formed of thermally insulated plastic materials orthe like and the housing base 14.1 has a recess 14.2 formed therein, theaccelerometer device having one end 12.1 mounted on the base so that anopposite end 12.2 of the device extends over the base recess to isolatethat end of the device from thermal stresses within the housing such asmight be due possible thermal expansion mismatch at thehousing/substrate interface. Preferably the unit has mounting holes 15for securing the device in use. Preferably also the unit is providedwith shock absorption means to reduce risk of damage to the beam.Preferably for example a sleeve 17 of compressible or resilient materialor the like such as rubber, polyurethane or styrofoam or the like isattached around the unit housing to avoid shock damage if the unit isdropped. The sleeve preferably covers unit corners but also covers theentire device if desired. Also if desired a resilient pad 19 is securedbetween the substrate and the base by adhesives or the like not shown tofurther reduce the risk of shock damage.

As shown particularly in FIGS. 2 and 3, the accelerometer device 12comprises an electrically insulating substrate 18 which is preferablyformed of a rigid ceramic material such as alumina or the like but whichis also adapted to be made from any of the rigid electrically insulatingmaterials commonly used in printed circuit board substrates and thelike. An electrically conducting surface such as copper or aluminum filmmetallizations or the like are provided on the substrate as shownparticularly in FIG. 2 to define a fixed capacitor plate or detect plate20, preferably near one end of the substrate 18, and to define acapacitor source plate connector 22 closely adjacent to the detectplate, the source plate connector preferably being located closer tosaid end of the substrate. Preferably the electrically conductiveportions also define circuit paths as indicated at 24 which areelectrically connected to the detect plate 20 and to the sourceconnector 22 to connect the detect plate and source plate connector inan electrical circuit. Preferably a metal shield layer 25 is provided onthe back side of the substrate as indicated in FIG. 1.

An electrically conductive metal plate member 26 of a stiffly resilientand preferably very flat material is provided with an attachment plateportion 28 at one end of the member, with a capacitor source plateportion 30 closely adjacent to the attachment plate portion, and withintegral beam 32 extending between the attachment plate and source plateportions of the member. Preferably the beam member comprise a first beamportion having a pair of beam elements 32.1 connected to respectiveopposite sides of the attachment plate portion by parts 32.2 of themember which are of relatively much smaller cross section. The pair ofbeam elements 32.1 extend in parallel relation to each other away fromthe attachment plate portion of the member to dispose distal ends 32.4of the pair of beam elements at a location spaced from the attachmentplate member portion 28. A second beam portion 32.5 is connected to thedistal ends of the first pair of beam elements and extends back towardthe attachment plate portion 28 of the member. If desired, theattachment plate portion is provided with rivet hole 32.6 or the likeand/or with solder regulating holes or slots 32.7. Preferably the platemember is formed of a metal material such as the metals commonly calledKovar, Invar or Alloy 42 (nominally 42% nickel, balance iron) having arelatively low coefficient of thermal expansion.

The attachment plate 28 of the member is secured in electricallyconductive relation to the capacitor source plate connector means 22 onthe substrate so that the capacitor source plate portion 30 of themember is normally disposed in a selected, originally, spaced relationto the capacitor detect plate 20 to form a device capacitor havingselected original capacitance and to permit movement of the capacitorsource plate 30 toward or away from the fixed detect capacitor plate inresponse to acceleration forces to modify that capacitance as a functionof the device acceleration. Preferably the metal member normally extendsin closely spaced relation to the substrate and has only a few thousandsof an inch spacing between the capacitor source and detect plates sothat the accelerometer device provides sharp change in capacitance withfast response to acceleration of the device and so that a film of airbetween the metal member and the substrate provides squeeze film dampingto limit vibration of the metal member in the device.

In one preferred embodiment of the invention as indicated in FIG. 4, ashim means 34 is disposed between the attachment plate portion of themetal member and source connector means 22 on the substrate to determinethe spacing of the capacitor source and detect plates. Preferably theshim means is soldered, brazed, welded or epoxied or the like to theattachment plate portion of the member and to the source plate connectormeans. Preferably the shim and member are secured as indicated at 36using a thin, strong, gold-tin solder having a relatively lowcoefficient of thermal expansion. In that arrangement, the solderregulating slots 32.7 aid in achieving a thin solder connection andpermit visual observation of the solder under the attachment portion ofthe member to assure proper extent of the solder connection.Alternately, if desired, rivet means 38 or other conventional attachmentmeans are used to secure the attachment plate portion 28 of the metalmember to the shim 34 and source plate connector means as shown in FIG.5. In another preferred embodiment of the invention, the metal platemember 26 is preferably etched, coined or machined so that the sourceplate portion 30 thereof is reduced in thickness preferably on bothsides, as indicated at 30.1, 30.2 in FIG. 6, whereby, when theattachment plate portion 28 of the member is secured to the source plateconnector means 22 with a thin, strong, gold-tin solder 36 as shown (orwith rivet means or the like), the space between the source plate anddetect plate of the device capacitor is precisely determined, the verythin solder thickness being relied on only to secure the member to thesubstrate so that very precise spacing of the capacitor plate isachieved.

In a preferred embodiment of the invention, stop means 40 are alsoprovided on the attachment portion of the metal member to extend overthe source portion of the member to limit movement of the source plate30 away from the detect plate 20 in response to device acceleration. Ifdesired, the stop is attached with solder 36 as shown in FIG. 6 or withthe rivets 38 and a shim 39 as shown in FIG. 5. Preferably the stopmeans comprises a stop plate 40.1 having a pair of legs 40.2 (only oneof which is shown in FIG. 6A) which are disposed at opposite sides ofthe source plate 30 as shown in FIG. 6A and secured to a metallizationpad or the like (not shown) on the substrate 18 by solder or rivet meansas will be understood. Preferably the stop 40 or 40.1 is spaced closelyover the source plate 30 to achieve some corresponding squeeze film airdamping of the metal member to limit member vibration. Preferably acover 41 of a polyamide material for example is secured to the substrateover the member 26 to exclude particles and other extraneous materialsfrom the member area during subsequent manufacture and calibration ofthe device and unit and during use of the unit over a long-service life.See FIGS. 1 and 3.

In one alternate preferred embodiment 12a of the invention as shown inFIG. 7, wherein corresponding features have corresponding referencenumerals a pair of source plate connector means 22a, 22b are providedalong a common line 19 with, and at either side of, the detect plate20a, and the metal member 26a has two attachment portions 28a, 28bconnected to the respective source plate connector means so that thesource plate 30a is movable along a vertical axis (extending into thepaper as viewed in FIG. 7) intersecting that common line.

Electronic means are connected to the circuit path means 24 to providethe accelerometer device with an electrical circuit capable of providingan output signal from the device corresponding to the deviceacceleration as will be understood. Preferable for example, anintegrated circuit unit 42 and device terminal pads 44 are provided onthe substrate 18 and connected to the circuit path means 24 in anyconventional manner. The device terminals are adapted to connect thedevice to a power source to apply a selected voltage to the devicecapacitor so that the device circuit provides an output signalcorresponding to the initial device capacitance and modifies that outputsignal as a function of device acceleration as movement of the sourceplate 30 modifies device capacitance in response to such acceleration.Preferably the integrated circuit is of known type adapted to becalibrated relative to device capacitance by electrical input to theintegrated circuit unit after the accelerometer device is mounted withthe accelerometer housing. That is, the housing cover is preferablyprovided with a port 46 aligned with the integrated circuit unit 42 sothat test probes (not shown) are connected to the integrated circuit inknown manner through the port 46. The device is then subjected to aselected acceleration force (by turning the device over e.g.) and thedevice is calibrated with the test probes relative to that accelerationforce. The probes are then withdrawn and the port is sealed with an RTVsealant or the like as indicated at 48 to provide a finishedaccelerometer unit 10. Leads 49 from the electronics (only one of whichshown) are typically led through a hole in the cover as shown and sealedtherein with the sealant 48 as indicated in FIG. 1 to connect the unitto a power source and to provide the desired output signal.

In a preferred method for making the accelerometer devices 12, a sheetof substrate material is processed as indicated at 50 in FIG. 8 todefine a plurality of detachably connected substrate sections within thesheet. Electrically conductive film means or the like are then depositedon all of the substrate sections at the same time to define a capacitordetect plate, a source plate connector means, and circuit paths on eachof the substrate sections as indicated at 52 in FIG. 8. A plurality ofmetal members 26 and electronic means 42 as previously described,together with shim means 34, stop means 40, terminals 44 and/or rivetmeans, are then deposited on each of the substrate sections as indicatedat 54 in FIG. 8, and all of the sections are then subjected toconventional wave soldering techniques or to brazing, welding orepoxying or the like at the same time as indicated at 56 in FIG. 8 forelectrically connecting the capacitor detect plate and the source plateto the circuit path means 24 on each of the substrate sections. Ifdesired, the shim means, metal members and stop means are also adaptedto be soldered (as previously described) in the same step in which thedetect plate and source plate connector means are soldered to thecircuit paths 24. The substrate sections 18 are then separated from eachother as indicated at 58 in FIG. 8 to provide a plurality ofaccelerometer devices 12 having very uniform properties. Preferably forexample, as shown diagrammatically in FIG. 9, a sheet 60 of ceramicsubstrate material is scribed as indicated at 60.1 to divide the sheetinto a plurality of detachably connected substrate sections 18. Thesubstrate sheet is subjected to conventional metal deposition and/oretching techniques to provide (as shown in FIG. 3) detect plates 20,source plate connectors 22, and circuit paths 24 on each of thesubstrate section at the same time and metal members 26 and electronicmeans 42 are disposed on each of the substrate sections as previouslydescribed. Shim means 34 and stop means 40 are also arranged on eachsubstrate section as previously described. In one preferred embodimentelectrically conductive metal terminal pins 45 are provided on a carrierstrip 45.1 extending in a common direction along the carrier strip andthe carrier strip is disposed on the substrate sheet so that selectedterminals are aligned with selected portions of the circuit paths 24(corresponding to the pads 44) on each of the substrate sections 18. Themetal members, shim means, stops, terminals and electronic means arethen simultaneously subjected to wave soldering, and after suchsoldering, the carrier strip 45.1 is removed from the terminals; and thesubstrate sections are separated as indicated at 62 along the scribelines to provide the accelerometer devices of the invention as indicatedat 12a in FIG. 9.

In that arrangement, each of the accelerometer devices is adapted to beprecisely and accurately manufactured at low cost, each device is ofrugged and reliable structure and is adapted to be easily mounted in andsealed in a unit housing, and each unit is also easily calibrated inknown manner.

It should be understood that although particular embodiments of theaccelerometer device, unit and method of manufacture have been describedby way of illustrating the invention, this invention includes allmodifications and equivalents of the disclosed embodiments fallingwithin the scope of the appended claims.

We claim:
 1. An accelerometer device comprising an electricallyinsulating substrate having electrically conductive surface portion fordefining a capacitor detect plate, a capacitor source plate connectormember adjacent to the detect plate, and circuit path means forproviding electric connection to the detect plate and the source plateconnector member and to be connected in an electrical circuit; anelectrically conductive metal plate member having an attachment plateportion secured in electrically conductive relation to the source plateconnector member, a capacitor source plate portion, and integral,resilient beam means for supporting and precisely positioning the sourceplate portion and extending between the attachment plate and sourceplate portion of the member, the beam means including first beamportions extending away from the attachment plate portion to disposedistal ends of the first beam portions at a location spaced from theattachment plate portion, and a second beam portion extending back fromthe distal ends of the first beam portions toward the attachment plateportion to dispose the capacitor source plate portion of the member inclose proximity to the attachment plate portion in selected spacedrelation to the detect plate to form a capacitor normally havingselected capacitance and to permit movement of the source plate memberportion to modify the capacitance in response to device acceleration;and electronic means connected to the circuit path means to provide anoutput signal corresponding to device acceleration.
 2. An accelerometerdevice according to claim 1 having shim means secured between theattachment plate portion of the member and the capacitor source plateconnector member for normally disposing the capacitor source plateportion of the member in said selected spaced relation to the detectplate.
 3. An accelerometer device according to claim 2 wherein the shimmeans is secured to the capacitor source plate connector and theattachment plate portion of the member with an attachment selected fromthe group consisting of solder, weld, braze and epoxy attachment means.4. An accelerometer device according to claim 2 wherein rivet meanssecure the shim means and the attachment plate portion of the member tothe capacitor source plate connector means.
 5. An accelerometer deviceaccording to claim 1 wherein the attachment plate portion of the memberis relatively thicker than the capacitor source plate portion of themember for normally disposing the capacitor source plate portion of themember in said selected spaced relation to the detect plate.
 6. Anaccelerometer device according to claim 5 wherein the attachment plateportion of the member is soldered to the capacitor source plateconnector member.
 7. An accelerometer device according to claim 5wherein rivet means secure the attachment plate portion of the member tothe capacitor source plate connector member.
 8. An accelerometer deviceaccording to claim 2 having a stop secured to the attachment plateportion of the member extending over the capacitor source plate portionof the member in spaced relation thereto for limiting movement of thecapacitor source plate portion of the member away from the detect platein response to device acceleration.
 9. An accelerometer device accordingto claim 8 wherein the shim means is secured to the capacitor sourceplate connector member and the attachment plate portion of the memberand the stop is secured to the attachment plate portion of the memberwith an attachment selected from the group consisting of solder, weld,braze and epoxy attachment means.
 10. An accelerometer device accordingto claim 8 wherein rivet means secure the stop, shim means andattachment plate portion of the member to the capacitor source platemember.
 11. An accelerometer device according to claim 5 having a stopsecured to the attachment plate portion of the member extending over thecapacitor source plate portion of the member in spaced relation theretofor limiting movement of the capacitor source plate portion of themember away from the detect plate in response to device acceleration.12. An accelerometer device according to claim 11 wherein the attachmentplate portion of the member is secured to the capacitor source plateconnector member and the stop is secured to the attachment source plateconnector with an attachment selected from the group consisting ofsolder, weld, braze and epoxy attachment means.
 13. An accelerometerdevice according to claim 11 wherein rivet means secure the stop andattachment plate portion of the member to the capacitor source plateconnector means.
 14. An accelerometer device according to claim 1wherein the first beam portions comprise a pair of resilient beamelements disposed at respective ends of the attachment plate portion ofthe member to dispose respective distal ends of the pair of beam metalelements at the location spaced from the attachment plate portion, andthe second portion comprises an additional resilient beam elementextending from the distal ends of the pair of beam elements toward theattachment plate portion of the member to dispose the capacitor sourceplate portion of the member in said close proximity to the attachmentplate portion and in selected spaced relation to the detect plate. 15.An accelerometer device according to claim 14 wherein the metal platemember has portions thereof relatively smaller than the ends of theattachment plate portion connecting a pair of resilient beam elements torespective ends of the attachment plate portion of the member forisolating the pair of beam elements from stress associated with securingof the attachment plate portion of the member to the capacitor sourceplate connector means.
 16. An accelerometer device according to claim 15wherein the attachment plate portion of the member is soldered to thecapacitor source plate connector means, and the attachment plate portionof the member has a plurality of openings spaced therein for permittingthe solder to extend through the openings to enhance uniform securing ofthe attachment plate portion of the member to the capacitor source plateconnector means.